I don't see the need to lie or get people's hopes up. If you're not interested, just say so. No shame in that, and it doesn't make you look "rude" or "cheap".
Generally speaking, if the organization finds that people just aren't interested in whatever product or fundraising technique they are using, they will figure it out and try something else. Girl Scout cookies are an example of something that's so popular that they're probably not going away anytime soon, but you might as well be honest if you're truly not interested.
Edit: OK, sorry, there was a tiny tinge of resentment in this post. As I said earlier, I do volunteer work (not with Scouts though) and that does include fundraising. Sometimes I get the run-around from my friends when I have to "hit them up". They don't want to say no to a friend, so they'll say things like, "Well, er... I need to ask my wife first, and then we'll get back to you" or "Not this time, but ask me again next year!" This is more aggravating to me than if they simply said "Sorry, no, I'm not interested" -- I won't be offended, honest! -- and at least with the latter response I can report back to our board of directors and say "Hey, nobody I asked was interested in this type of event, maybe we need to think of alternatives".